There are known transfer presses which include a transfer system for conveying workpieces to be pressed through a series of work stations in timed relation with performance of a series of pressing operations. Typically, the transfer system comprises a pair of feed bars juxtaposed so as to extend in a workpiece transferring direction and cross bars each spanned between these feed bars. The transfer system conveys workpieces held by vacuum caps by vacuum adsorption, these vacuum caps being attached to the cross bars. Alternatively, the transfer system conveys workpieces gripped at both sides by fingers attached to the feed bars. In this case, the pair of feed bars perform two-dimensional or three-dimensional movement to transfer an individual workpiece from one station where a set of dies are disposed to the next adjacent station where another set of dies are disposed.
The most typical method for driving the feed bars is a mechanical driving method in which the feed bars are driven, being linked to the press system with a cam and link mechanism. This method however reveals the disadvantage that the alignment of dies at the time of die replacement is extremely difficult and therefore the individual driving method becomes prevailing recently according to which the feed bars are driven with motors (servo motors) different from the motor for the press system.
There is the danger in such transfer presses having a transfer system driven by the individual driving method that the servo motor is brought into a free-running condition in the event of power failure during the operation of the feed bars, which would cause an interference, for instance, between the cross bars and the dies. To reduce the danger, such transfer presses are provided with a mechanism for rapidly bringing the servo motors to a stop in case of power failure.
This transfer system stopping mechanism incorporated in prior art transfer presses will be described with reference to FIG. 4. In FIG. 4, servo motors 50, 50' for driving feed bars (not shown) are connected to an a.c. power source 53 through their respective servo amplifiers 51, 51' and a converter 52. Each servo amplifier 51 (51') is provided with a control circuit 55 for outputting a control signal to a power transmission 54 for each servo motor 50 (50') to control the rotation of the servo motor 50 (50'). Each control circuit 55 is actuated in response to a command signal sent from a controller 56 and to a start signal from a relay logic 57. Each control circuit 55 is supplied with control power (400V) from the a.c. power source 53 and a capacitor 58 is connected to a point on the electric supply line of each control circuit 55 in order to back up the power source 53 in the event of power failure. A regenerative resistance 59 is connected to the converter 52, for consuming, as heat, regenerative energy that is generated during braking of the servo motors 50, 50'.
In the above circuit configuration, if the a.c. power source 53 is interrupted because of power failure, the supply of power to the electric supply line and therefore to the controller 56, control circuit 55 and relay logic 57 is interrupted. When the start signal from the relay logic 57 goes down, the servo amplifiers 51, 51' mask a command from the controller 56 and the internal logic is put in operation to stop the motors. Concretely, the regenerative energy that has been generated during braking of the servo motors 50, 50' is released to the regenerative resistance 59 as indicated by arrow E whereby the energy is consumed as heat to forcibly bring the servo motors 50, 50' to a stop. In order to allow the control circuits 55 of the servo amplifiers 51, 51' to perform the control for releasing the regenerative energy, the control circuits 55 are supplied with energy drawn (as indicated by arrow F) from the respective capacitors 58 for backing up the power source, until the motors are stopped. Accordingly, the servo motors 50, 50' can be rapidly stopped even in the event of power failure.
When power failure occurs, the press slide of the press system is braked for an emergency stop because no electric power is supplied. Accordingly, the press system and the transfer system are independently interrupted in the case of power failure. In this case, there is no problem if the press slide and the transfer system can be stopped at the same time. However, in reality, the transfer system is stopped before a stop of the press slide because the inertia force of the press slide is much greater than that of the transfer system. When such an emergency stop occurs, there is the danger depending on the stop position of the transfer system that the press slide would interfere with the transfer system because of the inertia of the press slide, resulting in damage to or destruction of the press dies or transfer system which are very costly.
Attempts to solve this problem are proposed in Japanese Patent Laid-Open Publications No. 5-324027 (1993) and No. 6-106271 (1994). These publications disclose transfer presses in which if a supply of electric power is interrupted due to power failure, regenerative power which has been generated in the main motor coupled to the flywheel of the press system is fed to the servo motors of the transfer system so that synchronous driving of the press system and the transfer system can be maintained even in the case of power failure.
The driving circuits for the transfer motors disclosed in the above publications are provided with an inverter for converting d.c. voltage into a.c. motor driving voltage. The d.c. voltage supplied to this inverter varies according to the types of the motors employed and it is therefore essential to equip the driving circuits with a converting system (DC/DC converter) for converting the d.c. voltage into optimum voltage. Such a converting system is very expensive, which is an obstacle to practical use of transfer presses of the type disclosed in the above publications.
The present invention is directed to overcoming the foregoing problems and it is therefore one of the objects of the invention to provide a transfer press which enables, with an extremely inexpensive arrangement, synchronous driving of the press system and the transfer system in the case of power failure.